Semiconductor packages incorporating leadframes are well known and widely used in the electronics industry for housing, mounting and interconnecting semiconductor devices. Well known types of semiconductor devices are connected to a leadframe and then encapsulated in a protective package envelope for a wide variety of applications. The semiconductor devices are generally integrated circuits (ICs) formed on a die or chip cut from a semiconductor wafer.
Currently known leadframes 2, 22, 42 are shown in FIGS. 1A to 1C respectively. Each of the leadframes 2, 22, 42 includes a plurality of electrically conducting leads 4 that are held together temporarily during package manufacture in a planar arrangement about a central opening 6. They are held together by expendable dam bars 8 which form a frame supporting the leads 4. A die mounting pad 10 is supported within the central opening 6 by tie bars 12 (which provide a mechanical connection only and are used only when the package is being manufactured) and fused leads 14 (which provide a mechanical and electrical connection). The leads 4 extend from a first end which joins to the dam bars 8 to an enlarged opposite inner end 20 adjacent the central opening 6 and spaced from the die mounting pad 10. The tie bars 12 and fused leads 14 that support the die mounting pad 10 are angled downwardly (as shown in FIG. 2), which is known as down setting, so that the die mounting pad 10 is vertically displaced below the plane of the leads 4.
A semiconductor device, such as an IC die 16 is attached to the die mounting pad 10 during the process of semiconductor package manufacture. The semiconductor device 16 is attached to the die mounting pad 10 by any one of the attachment methods known in the field, for example, by solder, adhesive or double sided adhesive tape.
After the semiconductor device 16 is attached to the die mounting pad 10, wire bonding pads 18 on top of the semiconductor device 16 are electrically connected to corresponding ones of the inner ends 20 of the leads 4. Fine conductive bonding wire 24 is used to make the electrical connection so as to connect power, ground and signals between the semiconductor device 16 and the leads 4.
Some of the wire bonding pads 18 serve a ground bonding function and so have to be grounded by connection either directly to the die mounting pad 10, so-called down bonding, or to a fused lead 14 electrically connected to the die mounting pad 10.
When wire bonding is complete, the bonded semiconductor device and leadframe assembly is encapsulated in a protective package envelope made of a mold compound 26, as shown in FIG. 2 for the leadframe 42 of FIG. 1C. The protective envelope may be made of any mold compound common in the field, for example, high density epoxy resin. The molding process can be any suitable molding process that is common in the field, such as transfer molding. After the assembly is encapsulated, the dam bars 8 and tie bars 12 are cut away.
In the arrangement in FIG. 1C, ground wires 28 extend from those wire bonding pads 18 on the semiconductor device 16 which serve a grounding function, directly to the die mounting pad 10. The interface between the mold compound 26 and the die mounting pad 10 is susceptible to delamination over time, which can cause the electrical connection between the ground wires 28 and the die mounting pad 10 to weaken or break. Delamination can be prevented by using selective silver plating on the die mounting pad 10, but the ability to selectively plate in this way is limited by the capability of leadframe suppliers in plating technology. Also, selective plating in this way customizes the leadframe too much. Delamination can also be prevented by using an adhesion promoter, but this increases the cost of producing the semiconductor package due to added processing of the leadframe.
In order to overcome the problem of delamination weakening the ground wire connection, it is also known to bond the ground wires 28 directly onto the horizontal portion of the fused leads 14, as is shown in FIGS. 1A and 1B. In FIGS. 1A and 1B, each fused lead 14 has a bonding region 14a co-planar with the leads 4 and a downset region 30 angled down from the plane of the leads 4. This can cause instability during wire bonding due to bonding near the downset or angled down region 30 of the fused leads 14.
FIG. 1D shows the pair of fused leads 14 at the top of FIG. 1A in more detail. Each fused lead 14 has two regions, a bonding region 14a, on a level with the leads 4 and a downset or angled down region 30. In the embodiment shown in FIGS. 1A and 1D the bonding region 14a is of increased surface area so as to provide a large enough area to which to bond the ground wires 28 in a stable manner. However, by increasing the size of the bonding region 14a of the fused leads 14, the overall length of the fused leads 14 is increased, leading to a wider central opening 6, which restricts the size of the die mounting pad 10 and so limits the size of the semiconductor device 16 which can be used in a semiconductor package incorporating the leadframe 2 of FIG. 1A.
The bonding region 14a in FIG. 1B has not been enlarged as in the FIG. 1A embodiment. This has the advantage that a larger semiconductor device 16 can be used in a semiconductor package incorporating the leadframe 22 of FIG. 1B.
However, the bonding region 14a may not be large enough for a reliable bond between the groundwires 28 and the bonding region 14a. 
Accordingly there is a requirement for a design of leadframe which enables good bonding for the groundwires without limiting the size of a semiconductor device which can be mounted on the leadframe.